Linear particle accelerator with collinear termination



y 1967 J. HAIMSON 3,319,109

LINEAR PARTICLE ACCELERATOR WITH COLLINEAR TERM INATION Original Filed June 29, 1961 "iiiirifliililwil mwmmiffi- 'I' GUN INPUT Q UNCHINQ Xx. YH z UNIFORM ACCELERATING TERMINATING COUPLER SECTION SECTION 3SECTION l5 FIG 2 2 INVENTOR. JACOB HAIMSON ATTORNEY United States Patent 3,319,109 LINEAR PARTICLE ACCELERATOR WITH COLLINEAR TERMINATION Jacob Haimson, Palo Alto, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Original application June 29, 1961, Ser. No. 120,597, now

Patent No. 3,264,515, dated Aug. 2, 1966. Divided and this application Apr. 18, 1966, Ser. No. 543,328

1 Claim. (Cl. 315-541) The present invention relates in general to particle accelerating means and more specifically to the construction of a linear accelerator having a collinear apparatus for attenuating radio frequency waves at the output end of the particle accelerating structure. This application is a division of application Ser. No. 120,597 tfiled June 29, 1961, now Patent No. 3,264,515.

In linear accelerators a source of particles such as an electron beam is directed down a slow wave structure which is adapted to propagate a radio frequency electromagnetic wave. The dimensions of the waveguide structure can be arranged such that there is an interaction between the traveling radio frequency wave and the electron beam whereby energy is transferred from the wave to the beam to accelerate electrons in the beam. At the end of the accelerating guide the accelerated electron beam can be directed onto an X-ray target for producing X-rays or can be passed out of the vacuum envelope and directed onto the object being irradiated. In optimizing the design of one type of accelerating structure which is to be utilized for X-ray production it has been found desirable to arrange for approximately ten percent of the radio frequency power to be remanent at the end of the accelerating waveguide after allowance for dissipation losses and beam loading. In other applications greater or less residual radio frequency power may result due to a variety of designs and operational requirements. For example, a greater amount of residual radio frequency power could be due to reduced beam loading caused by a reduction in beam peak current or the deph'asing of the radio frequency wave from a synchronous condition in order to reduce the terminal energy of the electrons. Less residual radio frequency power could result from increased beam loading. In particle accelerators for X-ray generation it is sometimes desirable to have even greater percentages of residual power at the end of the accelerating waveguide in the interest of output stability. This residual power is extracted from the accelerating waveguide and is either applied to an external load or fed back to the input of the accelerator. Such structures required a radio frequency transition at the end of the accelerating structure and may also require either an external load for dissipating the power or means for directing the radio frequency power back to the input. Also a radio frequency Wave permeable window may be necessary.

As Well as involving a great deal of expense, the additional structure necessary to dispose of the radio frequen-cy power requires a considerable amount of space. In many applications it is necessary to have the particle accelerator fit'in as small a space as possible. As a typical example, it is desirable to have particle accelerators which are used for therapy fit within as small a room as possible so that the accelerator can be placed in an existing room in a hospital. Since it is desirable to have the particle accelerator mounted on a gantry for irradiating a patient from many angles, the necessity for space around the end of the accelerating structure for terminating the radio frequency wave prohibits the use of a minimal size gantry.

Also when the residual radio frequency power is extracted at the end of the particle accelerator, the particle beam focusing structure is captured between the radio frequency wave coupling structures which are positioned at the ends of the accelerating structures. Since it is desirable to provide a vacuum envelope around the accelerating structure and inside the focusing structure, an assembly and maintenance problem exists. It is usually not possible to complete the vacuum envelope until the focusing structure is positioned around the accelerating structure. This vacuum envelope may have to be opened in order to remove the focusing structure for repairs.

According to the present invention, a termination is provided around the beam axis at the output end of a particle accelerator guide capable of dissipating the residual radio frequency power while permitting the transmission of the particle beam therethrough either without extracting energy therefrom, without altering the beam energy or by deliberately extracting energy therefrom. Such a structure is extremely useful in environments where cost and space are at a premium. Such a termination can be referred to as a collinear termination since it lies in a straight line with the accelerating section.

Therefore, the principal object of the present invention is to provide a termination around the beam axis at the output end of a waveguide structure for propagating a-radio frequency wave and capable of dissipating radio frequency power propagating therealong while permitting transmission of the electron beam therethrough and free thermal expansion of the accelerating structure.

A feature of the present invention is the provision of a particle accelerator assembly having a waveguiding structure adapted for passing charged particles therethrough and for propagating an electromagnetic wave therein, a collinear attenuator located at the output end of the waveguiding structure for attenuating residual power of the electromagnetic wave and a vacuum type envelope surrounding the waveguide structure with the waveguiding structure fixedly secured to the envelope at one position along the length of the waveguiding structure whereby the waveguiding structure is free .to expand within the envelope as the waveguiding structure becomes hot.

Other features and advantages of the present invention will become more apparent upon a perusal of the following specification taken in connection with the accompanying drawing wherein:

FIG. 1 is a schematic view of a particle accelerator utilizing the features of the present invention;

FIG. 2 is an enlarged side view, partially broken away of a portion of the structure shown in FIG. 1, and

FIG. 3 is a cross sectional view of a portion of the structure shown in FIG. 2 taken along the line 3--3 in the direction of the arrows.

Referring now to FIG. 1 of the drawing, there is shown a schematic view of an evacuated, linear, particle accelerator 10 adapted for accelerating a particle beam. While the particle accelerators shown and described below are especially designed for accelerating a beam of electrons, the features of the present invention are equally applicable to apparatus for accelerating beams of other particles'such as, for example, positrons. Also, the apparatus is adaptable for use with both pulsed and continuous beams.

The particle accelerator 10 includes a particle source 11 such as an electron gun and an accelerating structure including, for example, a bunching section 13, a uniform accelerating section 14 and a terminating section 15 all of which are adapted to pass the electron beam directed thereinto by the source 11.

Typically for pulsed operation of a particle accelerator a squared high voltage pulse is applied to electrodes within the gun assembly 11 which serve to pulse the emission of the electrom beam.

A high power, radio frequency source (not shown), for example, a klystron amplifier, serves to provide to the accelerating structure by means of an input coupler 12 peak radio frequency beam acceleration power as, by way of illustration, on the order of 1.75 megawatts at a certain high frequency as of, for example, 2,998 megacycles. The high frequency source is pulsed on in synchronism with the pulses applied to the electrodes of the electron gun.

In the bunching section 133 which can typically be a disc loaded wageguide, certain dimensions, such as the waveguide inside diameter, the beam aperture diameter, the disc spacing and the disc thickness, can be varied along the length thereof whereby the electron beam passing therethrough will be velocity modulated such that the beam will be formed into bunches of electrons as it passes into the uniform accelerating section 14. Also the bunching section may take a number of different forms, or may not even be utilized under certain conditions.

The accelerating section 14 is typically a length of disc-loaded waveguide forming a plurality of coupled cavities wherein the discs have a constant size aperture therein and are uniformly spaced along the length of the waveguide. In the buncher section 13 the electrons in the beam are bunched, and these bunches ride up to the crest of the radio frequency wave propagating through the accelerating structure. As these electron bunches pass through the uniform accelerating section 1 1, energy is continuously given up by the radio frequency wave to the electron bunches and the bunches are thereby greatly accelerated.

In typical linear electron accelerators when the accelerated electron beam emerges from an accelerating structure the particles making up the beam will have obtained extremely high energies such as anywhere from a few to very many million electron volts. Also, for maximum efliciency of X-ray production the radio frequency power remaining in the accelerating guide when the traveling radio frequency wave has reached the end of the accelerating structure may typically be on the order of of the injected radio frequency power. This power is usually passed through an output transformer and either through radio frequency vacuum window assembly to a water cooled load or through a waveguide to be fed back into the input of the accelerating structure.

The terminating section according to the present invention and more fully described below is positioned at the output end of the uniform accelerating section 14 and not only acts as a radio frequency wave terminating section but also as a means for continuing to accelerate the electron beam after it enters from the accelerating section 14 to the desired energy level, or leaving the entry energy unaltered or reducing the beam energy as required by the particular application. It is usually desirable to continue to impart energy to the beam in the terminating section.

The external wall of the waveguide of the bunching section 13, the accelerating section 14 and the terminating section 15 can, itself, constitute the vacuum envelope for the particle accelerator 111 or, as is shown in the illustrated embodiment of the present invention, a vacuum envelope 16 can enclose the radio frequency waveguide which is then provided with pump out holes whereby the waveguide can be evacuated.

The hi h energy bunched particle beam emanating from the terminating section of the particle accelerator 10 can either be passed through a continuation of the evacuated envelope or as shown for convenience directly through a particle permeable vacuum window into open atmosphere. In either case the particle beam can be bent, scanned, or directed onto a target for emitting Xrays, or uilized in any one or more of a number of known schemes.

A beam focusing solenoid 17 circumscribes the accelerating structure to prevent the beam from spreading as it travels along the length of the particle accelerator 111. Since with the present invention a radio frequency wave coupler is not provided at the output end of the particle accelerator 10, the focusing solenoid 17 can conveniently be slipped over the output end of the accelerating structure onto the accelerating structure. This is a most desirable advantage of the present invention and reduces the cost and complexity of the vacuum envelope 16. Another great advantage of this arrangement owing to the absence of external connections is the capability of the acceleration structure to freely expand and contract within the vacuum vessel as described in detail below.

Referring now to FIGS. 2 and 3 there is shown a typical embodiment of the terminating section 15 of the present invention. The structure of the terminating section 15 includes a disc loaded waveguide 18 forming a slow wave structure for the radio frequency power in the accelerating structure and having the same dimensions as the disc loaded waveguide of the uniform accelerating section 14. The loaded waveguide 18 is made up of a plurality of centrally apertured conductive disc members 19 as of, for example, copper, each disc member 19 being spaced from the adjacent disc member 19 by a hollow cylindrical spacer wall 21 of high, electrically resistive magnetic material such as, for example, magnetic stainless steel.

The alternatively stacked disc members 19 and spacer wall 21 form a plurality of cavity resonators 22 coupled together through the central apertures 23 in the disc members 19. A plurality of pump out holes 24 are provided in each of the spacer walls 22 to assist in the evacuation of the waveguide 18 positioned within the envelope 16.

Waveguide cooling means such as water cooling tubes not shown) are provided on the exterior surface of the waveguide for cooling the waveguide since a large amount of heat is generated in the terminating section wherein residual radio frequency power is attenuated. 1f the wall of the waveguide 18 were the vacuum envelope it could be completely surrounded by a cooling fluid. A plurality of radial supports 25 are provided for supporting the waveguide 18 within the envelope 16.

A radio frequency cut-off device 26 is provided at the end of the waveguide 18 of the terminating section 15 for preventing the passage of radio frequency waves through the end of the waveguide 18. However, this cut-off device 26 can be removed from the end of the guide for extracting or introducing radio frequency power during the testing of the accelerating structure.

The last cavity resonator 22 in the terminating section 15 provides a reflection for the radio frequency waves traveling along the accelerating structure whereby the residual radio frequency wave power at the end of the particle accelerator 11 is reflected back down the accelerating structure in a direction opposite to the direction of the particle beam.

By the construction of the particle accelerator described above the accelerating structure can be positioned within the vacuum envelope 16 with only the input end of the accelerator structure 13- anchored to the vacuum envelope 16 whereby the remainder of the accelerating structure including the length of the bunching section 13, the accelerating section 14, and the terminating section 15 is free to move lengthwise within the envelope 16 when the accelerating structure expands as it becomes hot.

The particle accelerator 10 operates in the following manner. As the radio frequency waves coupled into the accelerating structure through coupler 12 travel through the bunching section 13 and the accelerating section 14 the waves impart energy to the electrons passing therethrough to bunch and accelerate the particles of the beam. From the accelerating section 14 the radio frequency wave propagates into the terminating section 15 wherein it continues to interact with the particle beam and impart energy thereto. Furthermore, as the radio frequency wave propagates through the terminating section it is greatly attenuated, especially by the highly resistive spacer walls 21. Thus, the radio frequency wave continues to give up energy to the electron beam as it is being attenuated itself. In the last cavity resonator 22 of the terminating section 15 the radio frequency wave is reflected and travels backward in the terminating section 15 in the opposite direction to the direction of the electron beam and is again attenuated as it passes therethrough. The length of the terminating section is selected such that the attenuating characteristics of the power is approximately 1.75 megawatts and electron energies on the order of about 6 million electron volts are produced. However, much greater energies can be produced. The tables show the attenuating characteristics of particle accelerators of equal length wherein along the accelerator axis the lengths of the accelerating section 14 and the terminating section 15 are varied to vary the amount of attenuation presented to the residual radio frequency power.

TABLE I Forward Power, mw. Reflected Power, mw. Electron Distance along In Beam accelerator axis ems. Energy in Fig. 1 Unloaded Loaded, Unleaded Loaded (Mev 100 ma. pk.

1 Lo. 6% of forward power.

TABLE II Forward Power, mw. Reflected Power, mw. Electron Distance along In Beam accelerator axis ems. Energy in Fig. 1 Unleaded 'Loaded, Unloaded Loaded (Mev.)

100 ma. pk.

l I.e. 1.75% of forward power.

terminating section 15 in the forward direction plus the attenuating characteristics of the terminating section 15 on the reflected wave traveling in the backward direction provide the proper amount of attenuation for the residual radio frequency wave power entering the terminating section 15 from the accelerating section 1 lin order to prevent interference by the reflected Wave with the bunching and accelerating characteristics of the bunching section 13 and the accelerating section 14, and to prevent undue frequency pulling of the radio frequency generator.

In a typical particle accelerator of the type herein described the electron phase positions commence to deviate undesirably from their design orbits Within the buncher section when the available forward power is reduced by 8%. Also, the capability of the buncher section to produce an energy focus is effected when the reflected power is at a high level.

Even though the terminating section 15 attenuates the residual power of the radio frequency wave in the for Ward direction, under some conditions it is undesirable to have the residual radio frequency power at the end of the terminating section so low that the particle beam will regeneratively provide radio frequency power and will give up energy to the radio frequency structure. According to the present invention the length of the termination can be arranged such that the attenuation in the forward direction can be reduced to zero if necessary but for practical purposes this attenuation is selected such that in combination with the attenuation in the backward direc-. tion the level of the reflected power is sufficiently reduced whereby the bunching and accelerating characteristics of the accelerating structure are not impaired and the radio frequency generator is not pulled.

The following two tables show the manner in which the residual radio frequency power in a typical particle accelerator can be attenuated within the accelerating structure. The specific particle accelerator is especially adaptable for therapy. The injected peak radio frequency The amount of attenuation presented to the residual radio frequency power leaving the uniform accelerating section 14 can be varied in several different ways. Attenuation, I, is determined by the following formula:

wherein c is the velocity of light, Q is 21r times the ratio of the energy stored in the cavity resonators of radio frequency propagating structure to the energy lost in the cavity resonatols of the structure per cycle, Vg is the group velocity of the radio frequency wave traveling along the structure and is the free space wavelength of the radio frequency wave. Therefore, for a given operational frequency the attenuation in the terminating section 15 can be increased over the attenuation presented in the uniform accelerating section 14 by decreasing Q and/0r Vg. Q can be decreased by decreasing the ratio of effective cavity volume to effective electrical surface area. Another way of reducing the Q of a cavity is to increase the permeability and/or resistivity.

In the embodiment described above the cavity walls of high resistivity magnetic material, such as magnetic stainless steel greatly increased the attenuation of a terminating section having cavities of the same dimen sions as those of the accelerating section. For example, accelerating sections having copper spacer walls would have a Q on the order of 12,500 whereas the cavities of the terminating section 15 described above of similar size as those of the uniform accelerating section but with magnetic stainless steel Walls 21 would have a Q on the order of 1400. Even if the walls were of high resistivity non-magnetic material instead of magnetic material the Q would :still be on the order of 2500.

As an alternative embodiment of the present invention the disc members 19 as well as the spacer walls 21 can be made of high, electrically resistive material to provide even greater attenuation for the radio frequency wave in the terminating section 15. However, in such a structure the heat dissipated in the disc members 19 cannot be conducted out to the cooling means surrounding the waveguide 18 as easily as in the embodiment described above wherein the disc members 19 are made of highly conductive material such as copper. In this present embodiment cooling means such as water channels can be provided into the disc members 19 to more adequately cool these disc members 19.

Other constructions for the terminating section 15 such as described in the above-mentioned parent patent application of which the present application is a division are possible within the practice of the present invention. For example, other constructions can include the provision for uneven surfaces, a lower Q, an applied high electrical resistivity material and/ or a lower group velocity structure such as by means of reduced beam apertures and/or thicker discs.

Since many changes could be rnade in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

A particle acelerator comprising, in combination,

(a) a waveguiding structure adapted for passing charge particles therethrough and for propagating an electromagnetic wave therein for interaction with and acceleration of said charged particles;

(b) means other than said charged particles for supplying an electromagnetic wave to said waveguiding structure;

(c) said waveguiding structure having means at the downstream end thereof for attenuating any residual power of the electromagnetic wave not given up to said particles; and

(d) a vacuum tight envelope surrounding said waveguiding structure, said waveguiding structure being fixedly secured to said envelope only at the upstream end of the waveguiding structure, and means sup- 7 porting said downstream end of the waveguiding structure in said envelope for axial movement relative to the envelope, whereby said waveguiding structure is free to expand within said envelope as said waveguiding structure becomes hot, said vacuum tight envelope being surrounding by solenoidal focusing coil assemblies.

References Cited by the Examiner UNITED STATES PATENTS 7/1957 Bryant et al 3l3146 X 12/1962 Le Boutet et al. 315-5.42 

